1. Field of the Invention
The present invention relates to coated cemented carbide cutting tool inserts, particularly useful for milling of grey cast under wet conditions, preferably at low and moderate cutting speeds but also for milling of nodular cast iron and compacted graphite iron under wet conditions at moderate cutting speeds.
2. Background of the Invention
It is well known that for cemented carbide cutting tool inserts used in the machining of cast irons, the cutting edge is worn by different wear mechanisms such as chemical and abrasive wear but the cutting edge is generally also subjected to crack formation due to the intermittent cutting load, resulting in so called chippings and edge fractures caused by different types of cracks in the inserts.
Different types of crack patterns may appear during machining of cast irons. One important type is the so called comb cracks, which are formed perpendicularly to the cutting edge. The formation of comb cracks is strongly influenced by the cooling conditions during cutting. In particular, the use of fluid coolant increases the tendency to form comb cracks, often also called thermal cracks. The use of fluid coolant leads to large temperature gradients and thermal tensile stresses in the insert surface, increasing the tendency for formation of surface cracks, in particular in the case of coated cutting tool inserts where the hard but brittle ceramic surface coating is prone to crack under conditions involving unfavourable thermal tensile stresses. Cracks in the coating increases the risk for chipping and edge fractures and for flaking of the coating.
Characteristic for cast irons is the so called surface skin, the surface zone of the cast component often contains a structure which deviates considerably from the bulk structure and also contains hard inclusion and sand from the mould. In this case, a coated cemented carbide insert must be used including a substrate with the proper toughness of the cemented carbide grade and on the surface a wear resistant refractory coating.
Furthermore, different cutting conditions such as cutting speed, depth of cut, cutting feed rate and also external factors such as vibrations of the work piece and the above mentioned surface zone in iron casting, etc. , require a plurality of different properties of the cutting edge.
Commercial cemented carbide tool inserts for milling of cast irons under wet conditions are usually optimised with respect to one or two of the wear types observed.
U.S. Pat. No. 5,912,051 discloses a coated cutting insert particularly useful for dry milling of grey cast iron.
U.S. Pat. No. 5,863,640 discloses a coated turning insert particularly useful for intermittent turning in low alloyed steel.
In U.S. Pat. No. 6,062,776 is disclosed a coated cemented carbide cutting tool particularly designed for the wet and dry milling of workpieces of low and medium alloyed steels or stainless steels, with or without abrasive surface zones, in machining operations requiring a high degree of toughness of the carbide cutting edge. The external cutting conditions are characterised by complex shapes of the workpiece, vibrations, chip hammering, recutting of the chips etc.
In U.S. Pat. No. 6,177,178 is disclosed a coated cemented carbide cutting tool particularly designed for the wet and dry milling of low and medium alloyed steels.
WO 01/16388 discloses a coated insert particularly useful for milling in low and medium alloyed steels with or without abrasive surface zones during dry or wet conditions at high cutting speed, and milling hardened steels at high cutting speed.
It has now surprisingly been found that by combining many different features cutting tool inserts, preferably for milling, can be obtained with excellent cutting performance when milling grey cast iron using fluid coolant at low and moderate cutting speeds as well as in milling of nodular and compacted graphite iron using fluid coolant at moderate cutting speeds, in iron castings with or without cast skin.
The cutting tool inserts according to the invention show improved properties with respect to the different wear types prevailing at these cutting conditions as earlier mentioned.
The cutting tool inserts according to the invention consist of: a cemented carbide body with a relatively high W-alloyed binder phase and with a well balanced chemical composition and grain size of the WC, a columnar TiCxNy-layer, a xcexa-Al2O3-layer, a TiN-layer and optionally followed by smoothening the cutting edges by brushing the edges.
According to the present invention coated cutting tool inserts are provided consisting of a cemented carbide body with a composition of 7.3-7.9 wt. % Co, preferably 7.6 wt. % Co, 1.0-1.8 wt. % cubic carbides, preferably 1.4-1.7 wt. % cubic carbides of the metals Ta and Nb and balance WC. The average grain size of the WC is in the range of about 1.5-2.5 xcexcm, preferably about 1.8 xcexcm.
The cobalt binder phase is rather highly alloyed with W. The content of W in the binder phase can be expressed as the CW-ratio:
CW-ratio=Ms/(wt % Co xc3x970.0161)
where Ms is the saturation magnetization of the cemented carbide body in kA/m and wt. % Co is the weight percentage of Co in the cemented carbide. The CW-value is a function of the W content in the Co binder phase. A high CW-value corresponds to a low W-content in the binder phase.
It has now been found according to the present invention that improved cutting performance is achieved if the cemented carbide body has a CW-ratio of 0.86-0.94. The cemented carbide may contain small amounts,  less than 3 vol. %, of xcex7-phase (M6C), without any detrimental effect.
The coating comprises
a first (innermost) layer of TiCxNyOz with x+y+z=1, y greater than x and z less than 0.2, preferably y greater than 0.8 and z=0, with equiaxed grains with size  less than 0.5 xcexcm and a total thickness  less than 1.5 xcexcm preferably  greater than 0.1 xcexcm.
a layer of TiCxNy with x+y=1, x greater than 0.3 and y greater than 0.3, preferably xxe2x89xa70.5, with a thickness of 1-4 xcexcm, preferably 2-2.7 xcexcm, with columnar grains and with an average diameter of  less than 5 xcexcm, preferably 0.1-2 xcexcm.
a layer of a smooth, fine-grained (grain size about 0.5-2 xcexcm) Al2O3 consisting essentially of the xcexa-phase. However, the layer may contain small amounts ( less than 5 vol. %) of other phases such as xcex7- or the xcex1-phase as determined by XRD-measurement. The Al2O3-layer has a thickness of 1-2.5 xcexcm, preferably 1.2-1.7 xcexcm.
a further 0.5-1.0 xcexcm thick layer of TiN. This outermost layer of TiN has a surface roughness Rmaxxe2x89xa60.4 xcexcm over a length of 10 xcexcm. The TiN-layer is preferably removed along the cutting edge and the underlying alumina layer may be partly or completely removed along the cutting edge.
The present invention also relates to a method of making coated cutting tool inserts consisting of a cemented carbide body with a composition of 7.3-7.9 wt. % Co, preferably 7.6 wt. % Co, 1.0-1.8 wt. % cubic carbides, preferably 1.4-1.7 wt. % cubic carbides of the metals Ta and Nb and balance WC. The average grain size of the WC is in the range of about 1.5-2.5 xcexcm, preferably about 1.8 xcexcm. Onto the cemented carbide body is deposited
a first (innermost) layer of TiCxNyOz with x+y+z=1, y greater than x and z less than 0.2, preferably y greater than 0.8 and z=0, with equiaxed grains with size  less than 0.5 xcexcm and a total thickness  less than 1.5 xcexcm preferably  greater than 0.1 xcexcm using known CVD-methods.
a layer of TiCxNy with x+y=1, x greater than 0.3 and y greater than 0.3, preferably xxe2x89xa70.5, with a thickness of 1-4 xcexcm, preferably 2-2.7 xcexcm, with columnar grains and with an average diameter of  less than 5 xcexcm, preferably 0.1-2 xcexcm using preferably MTCVD-technique (using acetonitrile as the carbon and nitrogen source for forming the layer in the temperature range of 700-900xc2x0 C.). The exact conditions, however, depend to a certain extent on the design of the equipment used.
a smooth Al2O3-layer essentially consisting of xcexa-Al2O3 is deposited under conditions disclosed in e.g. U.S. Pat. No. 5,674,564. The Al2O3 layer has a thickness of 1-2.5 xcexcm, preferably 1.2-1.7 xcexcm.
a 0.5-1.0 xcexcm thick layer of TiN with a surface roughness Rmaxxe2x89xa60.4 xcexcm over a length of 10 xcexcm.
The smooth coating surface is obtained by a gentle wet-blasting the coating surface with fine grained (400-150 mesh) alumina powder or by brushing the edges with brushes based on e.g. SiC as disclosed e.g. in U.S. Pat. No. 5,861,210. The TiN-layer is preferably removed along the cutting edge and the underlying alumina layer may be partly or completely removed along the cutting edge.
The invention also relates to the use of cutting tool inserts according to above for wet milling using fluid coolant of cast irons such as grey cast iron, compacted graphite iron and nodular iron particularly grey cast iron at a cutting speed of 70-180 m/min and a feed of 0.1-0.4 xcexcm/tooth depending on cutting speed and insert geometry.